1. Field of the Invention
The present invention relates to a recording-sheet adsorbing apparatus for use in an image forming apparatus, such as an electrophotographic copying machine, having a recording-sheet adsorbing/transporting member, such as a transfer drum, and a toner-image carrier, such as an intermediate transfer belt, and more particularly to a recording-sheet adsorbing apparatus which is capable of adsorbing a recording sheet, to which a toner image must be transferred, with an attitude exhibiting excellent flatness and smoothness with respect to the recording-sheet adsorbing/transporting member and the toner-image carrier.
2. Description of the Related Art
An electrophotographic copying machine for performing so-called multiple transfer in such a manner that toner images are sequentially stacked on a recording sheet, such as copying paper, for example, as shown in FIG. 1, applies a transfer drum 100 for electrostatically adsorbing a recording sheet P to transport the same to a position at which a toner image is transferred.
Referring to FIG. 1, reference numeral 101 represents a photosensitive drum on which a toner image is formed by an electrophotographic process. Around the photosensitive drum 101, there are formed peripheral units, such as a charger, an image exposing unit, a toner development units for black, yellow, magenta and cyan images, a cleaning unit and the like (not shown). The above-mentioned peripheral units sequentially form electrostatic latent images of color-decomposed images on the surface of the photosensitive drum 101, and then develop the electrostatic latent images with toner in the corresponding colors. Thus, toner images in the color components are sequentially formed. The toner images on the photosensitive drum 101 are sequentially transferred to the surface of recording sheet P which is adsorbed to the outer surface of the transfer drum 100 and transported to the transferring position of the photosensitive drum 101.
The transfer drum 100 for adsorbing and transporting the recording sheet P is formed by winding a dielectric film 1 having a small thickness of about 150 .mu.m and made of PET (polyethylene terephthalate) or PVdf (polyvinylidene fluoride) around a drum frame 102 formed into a drum shape and rotatively disposed opposite to a transferring position for the transfer drum 100. On the outer and inner sides of the transfer drum 100, there are, at the positions as illustrated, disposed an adsorbing and pressing roll 2, an adsorbing corotron 3, a discharge restraining baffle 4, a transfer corotron 5, a separation corotron 103, a separation claw 104, a static-eliminating corotron 105, a cleaning unit 106 and a register roll 107. Referring to the drawing, reference numeral 6 represents a guide member for performing guiding to enable the recording sheet P to be introduced into the nip portion between the pressing roll 2 and the transfer drum 100. Reference numeral 108 represents a tie bar for connecting the right and left ring frames of the drum frame 102, and 109 represents an internally pushing member for upwards pushing the dielectric film 1 from the inside portion of the transfer drum 100 when the recording sheet is separated.
Among the above-mentioned elements disposed around the transfer drum 100, an element group consisting of the adsorbing and pressing roll 2, the adsorbing corotron 3, the discharge restraining baffle 4 and the guide member 6, or the like is called a "recording-sheet adsorbing unit". The adsorbing and pressing roll 2 forming the recording-sheet adsorbing unit is a roll made of a conductive material and having a cylindrical or columnar shape which is grounded and disposed to be brought into contact with the transfer drum 100 and separated from the same. The adsorbing corotron 3 supplies charges for adsorbing the recording sheet P to the outer surface of the transfer drum 100, that is, the charges having inverted polarity to that of toner which must be transferred are supplied to the inner surface of the transfer drum 100 (the dielectric film 1). The discharge restraining baffle 4 is arranged to restrain the discharge region of the adsorbing corotron 3 and disposed to be in contact with the inner surface of the transfer drum 100 (the dielectric film) in such a manner that it covers a portion of the adsorbing corotron 3.
The recording-sheet adsorbing apparatus having the above-mentioned structure is arranged in such a manner that the recording sheet P which is timed and transported from the register roll 107 is adsorbed to the transfer drum 100. That is, when the recording sheet P transported from the register roll 107 is introduced into a nip portion in which the rotating transfer drum 100 and the adsorbing and pressing roll 2 are in contact with each other, the adsorbing and pressing roll 2 presses the recording sheet P against the outer surface of the transfer drum 100. Moreover, charges having inverted polarity to that of the charges supplied by the adsorbing corotron 3 are injected into the sheet surface so that the recording sheet is electrostatically adsorbed to the outer surface of the transfer drum 100 (the dielectric film).
When the transfer drum 100 arranged to be rotated after it has adsorbed the recording sheet P reaches the transferring position opposite to the photosensitive drum 101, the inner surface of the transfer drum 100 is, by the transfer corotron 5, electrified with charged having the polarity opposite to that of the electrified toner on the photosensitive drum. Thus, the toner image on the surface of the transfer drum 100 is electrostatically transferred to the recording sheet P. Then, the transfer drum 100 is rotated, and then the inner surface of the drum and the transferred toner image are static-eliminated by the static-eliminating corotron 105. Then, the transfer drum 100 again transports the recording sheet P to the transferring position so that a toner image in next color is similarly transferred to the recording sheet P. Note that the adsorbing and pressing roll 2 and the cleaning unit 106 are separated from the transfer drum 100 in the second and following transferring cycles.
After the toner image of a final color component has been transferred to the surface of the recording sheet P, the separation corotron 103, the separation claw 104 and the internally pushing member 109 are operated. Thus, the recording sheet P to which toner images have been multi-transferred is separated from the transfer drum 100. The separated recording sheet P is transported to a fixing unit (not shown).
If the recording sheet P which is transported from the register roll 107 to the nip portion between the transfer drum 100 and the adsorbing and pressing roll 2 has absorbed water and thus the recording sheet P is made to be wet paper, leading end PS of the recording sheet P is sometimes waved perpendicular to the sheet transporting direction when it is transported as described above. If the waved recording sheet P is introduced into the nip portion, the thin dielectric film 1 of the transfer drum 100 is deformed attributable to an influence of the waved recording sheet P. Therefore, if waves are formed excessively, a non-adsorbed region b is, on the transfer drum 100, formed from an adsorbing region a of the recording sheet P after the recording sheet P has allowed to pass through the adsorbing and pressing roll 2. As a result, a defect in transferring the toner image sometimes takes place in the non-adsorbed region b. Note that the recording sheet is sometimes waved in the sheet transporting direction to a certain degree which sometimes causes the defective adsorption or the defective transfer to occur.
As techniques for preventing the defective adsorption and the defective transfer caused from waves of the recording sheet, there have been disclosed an apparatus (refer to Japanese Patent Laid-Open No. 2-157779) having an adsorbing and pressing roll formed into a barrel-like shape (a crown shape) having a larger diameter in the intermediate portion thereof to forcibly move the waves at the leading end of the sheet to the peripheral portions or an apparatus (refer to Japanese Patent Laid-Open No. 63-177181) arranged in such a manner that the circumferential speed of the adsorbing and pressing roll is reduced as compared with the speed, at which the recording sheet is transported, to prevent the slack of the sheet.
However, the above-mentioned two conventional techniques cannot attain a satisfactory effect. Especially, the former one of the conventional techniques causes the central portion of the dielectric film of the transfer drum to be deflected and deformed inwards by the barrel-like shape pressing roll. The state of deformation cannot be restored and removed until the sheet reaches the transferring position. Also the recording sheet is adsorbed with the shape corresponding to the deformed dielectric film. Thus, there arises a problem in that adsorption in the above-mentioned deformed state causes another defective transfer to take place. Moreover, the conventional recording-sheet adsorbing apparatus sometimes encounters a fact that waves at the leading end of the sheet are moved toward the rear end of the sheet when the sheet passes through the nip portion and accumulated waves form a great wave at the rear end of the sheet. Also the conventional technique cannot satisfactorily prevent the foregoing phenomenon.
To solve the defective transfer caused from waves of the recording sheet, also the applicant of the present invention has disclosed a technique arranged to provide a plate-like guide member having a special shape at both sides of a transporting passage of the recording sheet and in the vicinity of the nip portion with which a sheet guide passage for the recording sheet to reach the nip portion is temporarily bent in a direction apart from the transfer drum after which the passage reaches the nip portion (refer to Japanese Patent Laid-Open No. 5-46031). The foregoing structure is attempted to remove waves at the leading end of the recording sheet in a stage before the nip portion. Although the foregoing technique attains a certain degree of satisfactory advantage, the applicant has continued studies to suggest new solving means capable of obtaining the above-mentioned object.